Neil
It's some time back since I was involved in recruiting key people and things may have changed, but I would have thought the key is to prove that you have tried to find someone qualified and despite proof of your efforts you came up short.
Have you run adverts? What have you done to find someone?
Everyone has a unique skill set. If your Job Description just happens to match your guys skills/experience very closely

you may have a better chance. Proof of pro-actively looking and not getting the right candidates is crucial though.